Titanium dioxide has three crystalline types, anatase, brookite and rutile. Of these, the anatase and rutile types are manufactured industrially. As industrial methods of manufacturing titanium dioxide, there are a sulfuric acid method and a chlorine method, but in either case the anatase type is manufactured at first, and the rutile type is then obtained by heating and thus transforming the anatase type. Anatase type titanium dioxide (hereinafter merely referred to as “the anatase type”) has a specific gravity of 3.90 and a refractive index of 2.55, whereas rutile type titanium dioxide (hereinafter merely referred to as “the rutile type”) has a specific gravity of 4.20 and a refractive index of 2.70. Titanium dioxides of the both crystalline types are used in white paints, coating agents for paper, and colorants for plastics or rubber.
The anatase type is a crystallographically unstable form, and is pure white, showing virtually no optical absorbance up to 400 nm. The rutile type, on the other hand, is a stable form, is also more chemically stable than the anatase type and is highly robust after use, and has a slightly yellowish hue and is excellent in terms of tinting strength and hiding power. The anatase type and the rutile type each have their own characteristic features, and are used in accordance with these characteristic features. One usage of titanium dioxide is as a protective film on a substrate; in this case weather resistance and durability are required, and hence the rutile type is more suitable than the anatase type. Moreover, there are also cases in which such a film is used as an interference color film that makes any of various colors appear on a substrate through interference of visible light reflected at the film surface and the substrate interface. The rutile type is more suitable than the anatase type for such an interference color film, since the higher the refractive index, and the more dense and the more uniform, then the better the coloration.
For the above reasons, research has been carried out from hitherto into methods of manufacturing rutile type titanium dioxide films, but all such methods have involved first manufacturing the anatase type and then transforming this into the rutile type by heating. For example, there is a method in which a titanium tetrachloride (TiCl4) solution and a substrate are brought into contact with one another, and then in this state an alkali metal hydroxide is added to the solution, thus depositing anatase type titanium dioxide. In this method, tin oxide or iron oxide is put into the solution as a crystalline type transformation promoter, but even so heating to at least about 800° C. is necessary, resulting in the manufacturing process becoming complex and the manufacturing cost increasing. Furthermore, there are problems such as it not being possible to use the method on a substrate that cannot withstand the above temperature.
In view of such problems, it is an object of the present invention to provide a method of depositing rutile type titanium dioxide through a neutralization reaction, according to which heating for crystalline type transformation is essentially unnecessary and the manufacturing process is simplified, and thus costs can be reduced, and moreover rutile type titanium dioxide can easily be fixed even to a substrate having low heat resistance; furthermore, it is also an object of the present invention to provide a substrate, in particular glass flakes, to which rutile type titanium dioxide has been fixed using this deposition method.
To attain the above object, a first embodiment of the invention is characterized in that rutile type crystals are deposited through a neutralization reaction from a titanium-containing solution having a temperature in a range of 55 to 85° C. and a pH of not more than 1.3.
According to the first embodiment of the invention, crystals are deposited through a neutralization reaction from a titanium-containing solution having a temperature in a range of 55 to 85° C. and a pH of not more than 1.3, and hence rutile type titanium dioxide or a hydrate thereof can be deposited reliably.
A second embodiment of the invention is characterized in that tin or a tin compound is attached to a substrate.